Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 115

Many student neuroscientists are surprised when they learn that the most numerous cells in the nervous system are not neurons, but glia, which make up the nervous system’s connective tissues. In fact, glia outnumber neurons by about 10:1. Many types of glial cells with different functions exist (see Figure 3-4):

 Astrocytes are glial cells that form part of the blood-brain barrier and regulate ionic concentrations of extracellular fluid.

 Microglia scavenge for and clean up extracellular debris from dead cells and injury.

 Schwann cells and oligodendrocytes wrap axons to enhance signal propagation. (See the next paragraph for more.)

Figure 3-4: Types of glia cells (microglia, astrocytes, and myelin-producing Schwann cells and oligodendrocytes).

As the previous list shows, an important function of glial cells is that they wrap axonal membranes to enhance signal propagation. The glial cells that do this are Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system. These cells produce myelin, which is the fatty tissue made of lipids and lipoproteins that encloses certain axons and nerve fibers.

Both types of glial cells form a spiral, multilayered wrapping on a portion of an axon. This wrapping has two main effects:

 It greatly increases the membrane resistance (lowering leakage).

 It reduces membrane capacitance (refer to “Passive electrotonic conduction”) by increasing the distance between the fluid inside the axon and the fluid outside. The increase in membrane resistance prevents the signal from weakening, and the reduction in capacitance decreases the time smearing of the signal.

However, neither of these effects is enough to conduct action potentials for more than a few hundred micrometers without amplification, which is where repeaters come in. In glial-wrapped, or myelinated, axons, the glial wrapping contain breaks or gaps, approximately every millimeter. Located at these breaks, called nodes of Ranvier, is a high concentration of voltage-dependent sodium channels. The action potential jumps at high speed from node to node in a process called salutatory conduction. This conduction speed is about ten times faster than it is in unmyelinated fibers.